LOGOS Learning · 15
Water hammer on pump trip: the downsurge comes first
Water hammer on pump trip starts with a pressure drop, not a rise: the pump stops in the Mendiluce time Tp = C + K * L * v / (g * Hm), and if Tp < 2L/a the head at the pump falls by the full Joukowsky value dH = a * dv / g. On rising mains this can pull the hydraulic grade line below the pipe and cause column separation.
Trip the pump on a main that climbs to a reservoir. The water keeps moving up by inertia, the pump pressure collapses and a downsurge wave runs along the line.
Quick test
Three questions about this lesson. Got one wrong? The explanation shows right away.
1. Which is a typical protection against pump-trip surge?
2. On pump trip, the first effect at the discharge is…
3. A flywheel on the motor-pump set helps because…
Why this happens
Reference values from the water hammer calculator; the animation is the time simulation.
Pump run-down time (Mendiluce)
Without power, the pump and the water column slow down together. Long lines, with a lot of moving water, take longer to stop; high-head pumps stop faster.
Rapid or slow trip
As with valve closure, what decides is comparing the run-down time with the wave round trip, 2L/a. If the pump stops before the wave returns, the downsurge reaches the Joukowsky value.
The downsurge comes first
On a trip, the first move is a pressure DROP at the pump that runs along the line. On rising mains the grade line can fall below the pipe profile: negative pressure.
Column separation
If pressure nears vapor pressure, the water column breaks. When it rejoins, the impact is far larger than the predicted surge. The calculator uses −0.8 bar(g) as the margin. Usual protection: surge vessel, flywheel, surge tank and air valves.
Formulas in plain text
- Mendiluce formula (pump run-down time)
Tp = C + K * L * v / (g * Hm)- Tp = pump run-down time (s) · C = empirical coefficient from the slope Hm/L (1 for Hm/L < 0.20, down to 0 for Hm/L >= 0.40) (s) · K = empirical coefficient from the length (2 for short lines down to 1 for long ones) (-) · L = pipe length (m) · v = initial flow velocity (m/s) · Hm = pump head (m) · g = 9.81 m/s2
- Rapid or slow trip
Tp < 2 * L / a- a = pressure wave speed (m/s) · if true: rapid trip, full Joukowsky downsurge
- Downsurge at the pump (Joukowsky)
dH = -a * dv / g- dH = head change at the pump (m) · dv = velocity change, v0 to zero (m/s)
- Column separation check
p_min / (rho * g) = H_min - z < -8.2 m- below -8.2 m (-0.8 bar g, the calculator margin): column separation · H_min = minimum hydraulic grade (m) · z = pipe elevation (m) · rho = fluid density (kg/m3)
Frequently asked questions
What happens to the pressure when a pump trips?
The first effect is a pressure drop at the pump, because the water column keeps moving by inertia while the pump slows down. The downsurge travels along the line; later the column reverses, the check valve closes and an upsurge follows.
What is the Mendiluce formula?
It is an empirical estimate of the time a pump takes to stop after a power failure: Tp = C + K*L*v/(g*Hm). Long lines with a lot of moving water stop slowly, high-head pumps stop fast.
What is column separation in a pipeline?
When the pressure falls close to vapor pressure, the water column breaks and a vapor cavity forms. When the columns rejoin, the impact can be far larger than the predicted Joukowsky surge.
How do you protect a pipeline against pump trip water hammer?
Usual protections are a surge vessel (hydropneumatic tank), a flywheel to lengthen the run-down, a surge tank and air valves at the high points. The choice depends on the minimum pressure envelope along the profile.